• Journal of Astronomy and Space Sciences
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• 제28권3호
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• pp.217-223
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• 2011

Precise point positioning (PPP) is increasingly used in several parts such as monitoring of crustal movement and maintaining an international terrestrial reference frame using global positioning system (GPS) measurements. An accuracy of PPP data processing has been increased due to the use of the more precise satellite orbit/clock products. In this study we developed PPP algorithm that utilizes data collected by a GPS receiver. The measurement error modelling including the tropospheric error and the tidal model in data processing was considered to improve the positioning accuracy. The extended Kalman filter has been also employed to estimate the state parameters such as positioning information and float ambiguities. For the verification, we compared our results to other of International GNSS Service analysis center. As a result, the mean errors of the estimated position on the East-West, North-South and Up-Down direction for the five days were 0.9 cm, 0.32 cm, and 1.14 cm in 95% confidence level.

• 전력전자학회논문지
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• 제14권2호
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• pp.134-141
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• 2009

전원사고로 인하여 야기되는 전원급변상황에서 위상각의 새로운 PLL 방법은 추종속도를 개선시키기 위하여 제안되었다. 일반적으로 측정된 전원이 이상적이지 않고 고조파가 포함되어 있으며 센서의 노이즈 때문에 동기좌표계 PLL의 피드백 루프에는 LPF를 사용하고 있다. 그러므로 과도상태의 빠른 동특성을 얻기 위해서 LPF를 고려한 PLL시스템의 모델링을 제안하였다. 또한 전원의 급변 시에 빠른 위상각 검출을 위한 방법으로 자동으로 제어기 대역폭과 LPF의 차단주파수를 변동시키는 가변 파라미터 PLL방법을 제안하였다. 이를 시뮬레이션과 실험을 통해 검증하여 유효성을 보이고자 한다.

• Journal of Power Electronics
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• 제17권1호
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• pp.262-271
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• 2017

This paper studies the performances of five typical Phase-locked Loops (PLLs) for distorted and unbalanced grid, which are the Decoupled Double Synchronous Reference Frame PLL (DDSRF-PLL), Double Second-Order Generalized Integrator PLL (DSOGI-PLL), Double Second-Order Generalized Integrator Frequency-Lock Loop (DSOGI-FLL), Double Inverse Park Transformation PLL (DIPT-PLL) and Complex Coefficient Filter based PLL (CCF-PLL). Firstly, the principles of each method are meticulously analyzed and their unified small-signal models are proposed to reveal their interior relations and design control parameters. Then the performances are compared by simulations and experiments to investigate their dynamic and steady-state performances under the conditions of a grid voltage with a negative sequence component, a voltage drop and a frequency step. Finally, the merits and drawbacks of each PLL are given. The compared results provide a guide for the application of current control, low voltage ride through (LVRT), and unintentional islanding detection.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2010년 춘계학술대회논문집
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• pp.155-162
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• 2010

Recently, due to high oil prices and environmental pollution issues, interest of alternative energy development increases and the related research is widely conducted. Among those research activities the tidal stream power generation utilizes the tidal flow as its mechanical power resource and less depends on the environmental condition for installation and operation than other renewable energy resources. Therefore the amount of power generated is quite consistent and straightforward to predict. However, research on the tidal stream energy conversion turbine is rarely found. In the present study, two numerical methods were developed and compared for the open water Momentum Theory, which is widely used for wind turbines, was adopted. The moving reference frame method for Computational Fluid Dynamis solver were also used. Hybrid meshing was used for the complex geometry of turbines. The analysis results using each method were compared to figure out a better method for the performance prediction.

• 한국해양공학회지
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• 제23권1호
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• pp.16-23
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• 2009

The three-dimensional ship motion with forward speed was solved by a finite element method in the time domain. A boundary value problem was described in the frame of a fixed-body reference, and the problem was formulated according to Double-Body and Neumann-Kelvin linearizations. Laplace's equation with boundary conditions was solved by a classical finite element method based on the weak formulation. Chebyshev filtering was used to get rid of an unwanted saw-tooth wave and a wave damping zone was adopted to impose a numerical radiation condition. The time marching of the free surface was performed by the 4th order Adams-Bashforth-Moulton method. Wigley I and Wigely III models were considered for numerical validation. The hydrodynamic coefficients and wave exciting forces were validated by a comparison with experimental data and the numerical results of the Wigley I. The effects of the linearization are also discussed. The motion RAO was also checked with a Wigley III model through mono-chromatic and multi-chromatic regular waves.

• 한국전산유체공학회지
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• 제20권4호
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• pp.1-6
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• 2015

Centrifugal pumps consume considerable amount of energy in various industrial applications. Therefore, improvement of the efficiency of these machines has become a major challenge. Cavitation is a phenomenon which decreases the pump efficiency and even causes structural demage. Hence, the goal of this paper is to investigate the cavitation problem in the single-stage and double-stage centrifugal pumps. The Volume of Fraction (VOF) method has been used for the numerical simulations together with Rayliegh-Plesset model for the gas-liquid two-phase flow inside the pump. In order to capture the turbulent phenomena, the standard k-${\varepsilon}$ turbulence model has been adopted, and the simulations have been done as unsteady cases. In addition, the motion of the rotating parts has been simulated using Multi Reference Frame(MRF) method. The results are presented and compared in terms of hydraulic head and NPSH for both the single-stage and double-stage pumps. The H-Q curves show the effects of cavitation on decreasing the pumps performances.

• Smart Structures and Systems
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• 제21권5호
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• pp.611-621
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• 2018

Compared with the highway bridges, the relatively higher requirement on the safety and comfort of vehicle makes the high-speed railway (HSR) bridges need to present enhanced dynamic performance. To this end, installing a health monitor system (HMS) on selected key HSR bridges has been widely applied. Typically, the HSR takes fully enclosed operation model and its skylight time is very short, which means that it is not easy to operate the acquisition devices and download data on site. However, current HMS usually involves manual operations, which makes it inconvenient to be used for the HSR. Hence, a HMS named DASP-MTS (Data Acquisition and Signal Processing - Monitoring Test System) that integrates the internet, cloud computing (CC) and virtual instrument (VI) techniques, is developed in this study. DASP-MTS can realize data acquisition and transmission automatically. Furthermore, the acquired data can be timely shared with experts from various locations to deal with the unexpected events. The system works in a Browser/Server frame so that users at any places can obtain real-time data and assess the health situation without installing any software. The developed integrated HMS has been applied to the Xijiang high-speed railway arch bridge. Preliminary analysis results are presented to demonstrate the efficacy of the DASP-MTS as applied to the HSR bridges. This study will provide a reference to design the HMS for other similar bridges.

• The Journal of Asian Finance, Economics and Business
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• 제5권4호
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• pp.117-125
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• 2018

As various types of information technology systems are becoming more pervasive than ever, many studies have evaluated the systems from the user perspective. Some of them have used surveys to measure consumers' cognitive responses to the target technology. However, this method may cause problems if the survey participants do not have a useful frame of reference for evaluating an unfamiliar system. To examine this issue, the current study empirically tested the effect of personal product knowledge on the predictability of a behavioral model, such as Technology Acceptance Model. A series of measurement invariance tests as well as multi-group comparison tests were conducted for rigorous examination of the data. Our analysis showed that the variance of attitude that is explained by the two believes (perceived usefulness and ease of use) was relatively small when the survey respondents had lower amount of product knowledge. Moreover, the group had weaker causal relationship between attitude and intention to use the technology, hindering the predictability of the research model. The results indicated that respondents should have a certain amount of knowledge of the target system in order to form accurate beliefs and behavioral decisions. The findings of this study provide important implications on sampling strategies for researchers with new technology.

• 대한조선학회논문집
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• 제51권2호
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• pp.162-170
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• 2014

A CFD(Computational Fluid Dynamics) analysis is presented to predict hydrodynamic characteristics of a marine propeller. A commercial RANS(Reynolds Averaged Navier-Stokes equation) solver, namely FLUENT, is utilized in conjunction with fully unstructured meshes around rotating propeller. Mesh generation process is greatly accelerated by using fully unstructured meshes composed of both isotropic and anisotropic tetrahedral elements. The anisotropic tetrahedral elements were used in the flow domain near the blade and shaft, where the viscous effect is important, having complex shape yet resolving the thin boundary layers. For other regions, isotropic tetrahedral elements are utilized. Two different approaches simulating rotational effect of the propeller are employed, namely Moving reference frame technique for steady simulation, and Sliding mesh technique for unsteady simulation. Both approaches are applied to the propeller open water (POW) test simulation. The current results, which are thrust and torque coefficients, are compared with available experimental data.

• 대한기계학회논문집
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• 제12권1호
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• pp.95-105
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• 1988

본 연구에서는 실험적인 모우드해석법을 실제의 테니스 라켓에 적용하여 모우드파라미터를 구한 다음, 그 파라미터의 변화에 따르는 안정타점영역의 변화를 컴퓨터 시뮬레이션을 통해서 에측하였다. 또한 안정타점영역을 확장시키고 라켓의 동특성을 개선시킬 수 있는 모우드파라미터를 찾아서 라켓의 설계제작단계에 정보를 제공하는 테니스 라켓의 CAD(Computer Aided Design) 화를 위한 자료 획득을 목적으로 한다.